Excitonic Coupling of a TADF Assistant Dopant and a Multi-Resonance TADF Emitter

The excitation energy transfer (EET) between two conformers of the deep-blue thermally activated delayed fluorescence emitter DBA-BTICz and the multi-resonance fluorescence emitter ν-DABNA is studied by means of quantum chemistry according to Fermi's Golden Rule. Excitation energies, fluorescence and intersystem crossing rate constants of the individual EET donor and acceptor molecules, determined by a combination of density functional theory and a multireference configuration interaction approach, match the experimental data very well. Interestingly, two different conformers of DBA-BTICz with similar absorption, but distinct emission and transfer properties are found. The vibronic envelopes of the DBA-BTICz emission and the ν-DABNA absorption spectra are computed by means of a vertical Hessian approach. Their overall shapes and peak positions agree well with the experimental data but the widths of the computed vibronic spectra remain a critical factor in the evaluation of the spectral overlap integral. The distance and orientation dependencies of the excitonic coupling matrix elements, evaluated in the ideal dipole approximation (IDA), are carefully assessed by means of the monomer transition density (MTD) approach. Deviations between the IDA and MTD results of at most 20% at typical Förster radii justify the use of the computationally much less demanding IDA for estimating environmental and orientation effects on the EET rate.